In the fluid catalytic cracking (FCC) process for heavy petroleum fractions, the preheated high molecular weight hydrocarbon liquids and vapors are contacted with hot, solid catalyst particles and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons. In the FCC process, some non-volatile carbonaceous material or "coke" is deposited on the catalyst particles which reduces the effectiveness of the catalyst. Regeneration of the catalyst is achieved by burning the coke deposits from the catalyst. The hot gases resulting from burning the coke are passed through at least one cyclone separation means to remove the entrained regenerated catalyst. The regenerated catalyst is returned to the process while the cleaned hot gases are, typically, passed through a power recovery system.
A major problem experienced by users of power recovery equipment is process upsets. These process upsets may be the result of feedstock-to-catalyst pressure differentials, afterburn quench pressure build up, depressurization of the regenerator and other malfunctions which generally occur during new FCC plant startups but can occur at other times as well.
The only process upset which seems to be detrimental to the power recovery expander is a catalyst load dump which is caused by a rapid pressure drop in the regenerator. Some of the mechanisms which can cause this to happen are: (1) rapid opening of the bypass valve; (2) failure of the bypass valve control signal; (3) failure of signal lines to the bypass or throttle valve; (4) rapid opening of the throttle valve; (5) lack of sequencing communication between the throttle and bypass valves; and (6) backflow of feedstock into the regenerator causing rapid combustion and subsequent water quench. When a dump occurs, the whole fluidized bed is generally lost because once the cyclone dip tubes are exposed, all the catalyst goes. The amount of catalyst dumped is a function of the regenerator size and may vary from 20 to over 100 tons during a 15 to 30 minute time period. Not all of the catalyst goes through the expander, the majority of it goes through the bypass line into the waste heat boiler. When this happens, the catalyst goes out the stack of the waste heat boiler producing a large dust cloud which settles out over the area, much like volcanic ash. Settling out may take several days under proper atmospheric conditions. Since the catalyst is siliceous, such as silica-alumina, the catalyst passing through the expander results in severe erosion of a single stage expander and blockage of the second stage of a two-stage expander with the resultant wear down of the first stage rotor blades. Although the FCC process is under computer control, catalyst load dumps are not identified until after they have taken place and the expander is damaged.